This invention relates generally to are fault circuit interrupters, and more particularly to an arc fault circuit interrupter that uses the random start time and/or the duration of arc noise to detect the presence of arcing.
The demand for arc fault circuit interrupters that reliably detect the presence of actual arcs, while discriminating against similar phenomena such as the noise created by lamp dimmers, motor operated appliances and the like, is high. Arc fault circuit interrupters have application in circuit breakers to protect possibly long electrical circuits powering a number of devices, as well as in wiring devices such as receptacles to protect a single device or appliance. Especially in the case of wiring devices, the complexity and therefore the cost of an arc fault circuit interrupter is an important consideration. Simple are fault circuit interrupters that can be manufactured at a reasonable cost are likely to be much more widely used than more expensive designs.
We have observed that one characteristic of the noise produced by arcs is that the width of the noise pulses varies from cycle to cycle. This characteristic distinguishes actual arc noise from the noise produced by devices such as lamp dimmers, which are far more regular.
We have found that by comparing the lengths of successive noise pulses on a power line, and generating a fault when a predetermined number of pairs of unequal length pulses is detected, arc faults can be reliably detected, even in the presence of other noise producing, phenomena.
It is an object of this invention to provide an arc fault circuit interrupter that employs an electrical circuit that is simple enough, inexpensive enough and small enough to be included in wiring devices. It is another object of this invention to provide an arc fault circuit interrupter that is sensitive to arc faults, but resistant to false tripping, due to noise created by lamp dimmers and the like.
It is another object of this invention to provide an arc fault circuit interrupter that is sensitive to relatively low amplitude series arc faults of at least 5 amps of arc current, typically in series with the load and commonly referred to as Type A arc faults.
It is another object of this invention to provide an arc fault circuit interrupter that detects parallel or line to line arcs, producing currents of 75 amps or more, commonly referred to as Type B arc faults.
Briefly stated, and in accordance with a presently preferred embodiment of the invention, an arc fault detecting circuit using a pulse width modulation, or PWM, technique is used to sense series or parallel arc faults. When are faults occur, typically an arc step in current is followed by broad band arc noise which is caused by the random fluctuations in arc column resistance. In this invention, a logic signal is created which has a duration or width that corresponds to the time interval during which the broad band arc noise generated by the are is present. Typically an arc fault randomly starts, generates broad band arc noise, and then extinguishes as the next current zero cross of the AC load current is approached. The random start causes PWM of the logic pulse whenever the start of the arc randomly moves with respect to the current zero cross. An arc fault that randomly starts, and then randomly stops, will also cause PWM with respect to the zero cross. The PWM logic pulse is coupled to the input of a microprocessor which measures and stores the length of the present PWM pulse under measurement, and compares that pulse to the pulse length of the last stored PWM pulse, and increments a counter if the difference in pulse length is greater than a predetermined amount. When the counter number is greater than or equal to a predetermined number held in the microprocessor memory, an output signal is sent to a controller which activates the circuit interrupter.
Another input to the microprocessor, corresponding to the magnitude of the arc fault current broadband noise, is used to adjust the predetermined number value down or up depending on the intensity of the arc fault for faster or slower interrupts respectively.
This invention also provides discrimination against the noise produced by light dimmers, Power Line Carrier transmissions signals, motor noise, and similar phenomenon. Light dimmers do not produce arc noise, nor is there PWM of the conduction period, which stays essentially constant from half cycle to half cycle, and this type of signal is rejected by the present device. Power Line Carrier signals, or PLC, are rejected by the RF bandpass filter, by the bandpass below the frequency band of PLC transmissions and above the typical 60 HZ line current harmonics. PLC is also rejected by the nature of the typical PLC envelope signal which is not pulse width modulated. Motor brush noise is typically synchronous with the power line frequency and does not contain any pulse modulation of the motor noise interval and is also rejected by this invention. Motor noise is also typically of a lower magnitude than arc fault noise, which also aids in the rejection of motor noise.